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re: a simple solar collector
5 aug 2004
gary   wrote:

>>>i agree that if i only want to heat the garage, it would be better to 
>>>just open the door all the way...
>> 
>> warm air rises, but it seems to me you still need the screen to avoid
>> storing sun in the concrete floor, and maybe to reduce the amount of
>> light and radiant heat when you are working in the garage.
>
>your idea is that the screen would absorb some of the light, heat up, 
>and cause air around to rise to the ceiling where it could be fanned 
>into the house?

yes, with the fan mounted high on a wall between the house and the garage,
blowing air into the house. the screen might absorb most of the light...
full sun is 10k fc, but you only need about 50 for working, ie 2% of that.
you might hang one or two layers of 80% (absorbing) black greenhouse
shadecloth inside the garage door (it shrinks about 30% in boiling water.)
stuppy (800) 733-5025 sells 80% sundown polypropylene shadecloth in 5, 6,
10 and 12' rolls for about 14 cents/ft^2. you can sew it together with
fishing line and hang it with grommets or "poly clips." stuppy can make
a nice custom curtain with grommets for another $100 or so.

>i wonder if the screen could roll up like a window shade?  then the 
>screen might be able to basically control whether heat went mostly 
>into the garage (screen up), or mostly into the house (screen down)?

you could do that, but it seems easier to just slide it along a wire or
turn off the fan or raise its thermostat temp to make the garage warmer.
thought about storing heat in some pipes or poly film water ducts under
the garage ceiling? they might lie on a layer of foil with welded-wire
fence underneath. the foil would help keep the heat near the ceiling.

>> ...so we have something like this, viewed in a fixed font: 
>> 
>>             t
>>      ---    |  1/130
>> |---|-->|---*---www--- 40
>>      ---    |
>> 29.3k btu/h |  i -->
>>              ---www--- 70
>> 
>> 29.3k btu/h = (t-40)130 + 231(t-70)^1.5, or t = 70 + ((265-t)/1.78)^0.666
>> 
>> plugging t = 100 f into the right side makes t = 182 on the left.
>> plugging t = 182 in on the right makes t = 82.9 on the left.
>> repeating makes t = 91.8, then 91.1... not too hot.
>> you might collect 22.4k btu/h of useful heat
>> with a 76% efficiency... 
>
>> you might redo the calcs as above.
>
>i did this, and it looks pretty good -- tcol varys in sensible ways 
>with the sun input, and ambient temperature.

good...

>your equation: q = 16.6x5.25xsqrt(7(t-70)) cfm
>is this for a chimney?  is the 16.6 empirical or?

yes. q = 16.6asqrt(hdt) cfm is an empirical "stack effect" chimney formula.

>i guess that on the actual system, i could vary the 5.25 ft^2 gap area,
>and/or introduce flow restrictions if i wanted somewhat higher 
>tcol at the cost of lower efficiency?

yes. or build another hotter collector inside the outer one, or
collect warmer air on the north side of the mesh and cooler air
on the south side, next to the glazing.

>on cabinet doors that have a central panel surrounded by a frame, the 
>central panel expands across the grain, while the frame expands with 
>the grain.  there is much more expansion across the grain, and to keep 
>the panel from buckling or cracking, it floats in a dado in the frame. 
>  i wonder if the same technique could be used for these garage door 
>glazed panels -- i.e. the pc glazing panel would float in a dado in 
>the frame, perhaps with light caulking to prevent infiltration?

it might easily buckle... 0.020" flat polycarbonate is more flimsy than glass.

>i am thinking of the frame breaking the glazing up into panels that 
>are probably no more than 2 ft by 4ft ish.

or 2'x2'. more wood and work...

>not sure what you meant by using the wood to act as a spring?

picture a 4'x8' sheet of flat polycarb in a trampoline frame, always
in tension. if we attach it to a light wood frame on a hot day, it might
make the wood bow inwards when it gets cold, if the fasteners don't tear.
the wood frame might be mounted behind some face boards that let the
inner frame move but don't allow much air to leak around the gaps
formed by the bowing.

>anyway, it seems like i could go ahead and build the glazed panels/doors,
>and then experiment with gaps, screens, dividers, ducts and fans.

sounds like a plan. (steve baer would add "throw away your calculator.")

>i ordered one of the testo 405's that mentioned in another post -- 
>looks like it should make velocity measurements a lot easier.

i ordered one too, altho i already have a $400 dwyer version that's
larger and less sensitive. i found the testo in an sbse "agents of change"
tool kit which also contained a "pocket bolometer" (a 30 gallon plastic
trash bag :-), some hobos, a compass, a kestrel 3000 pocket weather meter
(airspeed, temp and rh, $145 from www.forestry-suppliers.com), a sylvania
light meter, a raytek mt4 ir thermometer, a solar transit template,
a pilkington sun angle calculator, a motorola flicker checker (a spinning
top with a pattern that shows whether fluorescent lamps have magnetic
(120 hz) or electronic (20 khz?) ballasts), a "globe thermometer" (a mat
grey ping-pong ball with a hole that slips over a thermocouple, used to
calculate mean radiant temperature), and a long-life plastic soap bubble
solution from toys-r-us.

testo says the hot wire should last for years, but it isn't replaceable,
so i wouldn't use it to stir coffee or to pound nails. testo considers
this a $150 "disposable instrument" :-)

nick




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